A. FIELD OF THE INVENTION
This invention relates to a method of treating a subterranean formation of non-uniform permeability, and more particularly concerns a method of reducing the permeability of subterranean formations to water thereby promoting better control of fluid injection patterns in the secondary or tertiary recovery of hydrocarbons and achieving water reduction in producing wells and thus reducing the quantity of water recovered from a subterranean formation penetrated by a well bore whereby the relative production rate of the hydrocarbons is increased.
B. DESCRIPTION OF THE PRIOR ART
Since only a portion of the oil contained in a subterranean reservoir can be recovered by primary methods, it has become general practice to employ various secondary or tertiary recovery techniques to produce the additional quantities of oil not economically recovered by primary methods. Of the various secondary or tertiary recovery methods available, one of the most widely practiced techniques is the displacement of oil from the reservoir with a driving fluid such as a floodwater injected for that purpose. Normally, in carrying out the flooding process, input or injection wells are utilized. These wells can be old existing wells or can be wells which are newly drilled into the oil-producing strata. The injection wells locations with reference to the production wells are selected to afford a desired flood pattern, the selected pattern depending in part upon field conditions, the locations of existing wells, and the operator's preference. Aqueous drive fluids, such as water, brine, or a viscous aqueous fluid are forced into the input wells under pressure, and out into the surrounding oil bearing strata towards the producing well or wells. While waterflooding has been rather widely practiced in recent years, it is not without considerable operating problems and economical limitations particularly those associated with low oil recoveries in proportion to the amount of water injected. Various surfactant and solvent floods have been proposed as means for recovering additional quantities of oil over that recoverable by conventional waterflooding. However, these processes face serious operating problems when practiced in heterogeneous formations containing strata or channels having permeability substantially higher than the bulk of the formation.
One of the major problems encountered in a flooding operation is the breakthrough of the flooding medium from the flood front to the producing well relatively early in the displacement process and rapidly increasing producing water/oil ratios following the initial breakthrough. These difficulties result from the displacing medium channeling or fingering through the oil-bearing structure to the producing well, thus bypassing large zones of the oil-bearing strata. The reason for the channeling of the flooding medium to the producing wells and the resulting low oil recovery is due in part to the peculiar structure of the oil-bearing strata. Underground oil reservoirs, in most cases, consist of layers of sand or rock and, since no reservoir rock is perfectly uniform in composition and structure, the permeability will vary across the rock face or strata. Also, fractures, cracks, and other abnormalities can promote channeling of the displacement of the fluid.
In the normal flooding operation, maximum oil recovery is obtained when the driven fluid goes up in a wide bank in front of the driving fluid which moves uniformly towards the producing well. To keep this bank of oil intact and constantly moving towards the producing well, a substantially uniform permeability must exist throughout the strata. If this uniform permeability does not exist, or is not provided, the flooding fluid will seek the areas of high permeability, and channeling occurs with the subsequent loss of some driving fluid energy and the appearance of excessive amounts of driving fluid in the producing well. Moreover, as the more permeable strata are depleted, the driving fluid has a tendency to follow channels and further increase the consumption of the flooding medium to the point where the process becomes uneconomical. It is, therefore, desirable to operate at a drive fluid to oil ratio that is as low as possible.
Another problem associated with the production of oil from oil-bearing formations containing highly permeable water channels communicating the production well with the water zone is the intrusion of water into the well. Not only does this water intrusion cause production and disposal problems, but more importantly the beneficial effect of the natural water drive is at least, in part, lost thereby adversely affecting oil recovery.
It is advantageous to reduce the permeability of the water channels so as to render the formation more uniformly permeable and to increase the unit efficiency of the water drive, or alternatively to shut off the water intrusion.
Many processes have been proposed for reducing the permeability of a subterranean formation. For instance, U.S. Pat. No. 3,762,476 discloses that the quantity of water recovered from a subterranean formation penetrated by a well bore can be reduced by injecting into the subterranean formation a first thickened aqueous solution, a complexing ionic solution of multivalent cations and retarding anions, a brine slug, and a second thickened aqueous solution. Complexing ionic solutions disclosed in the patent have a multivalent metal cation selected from the group consisting of Fe.sup.2+, Fe.sup.3+, Al.sup.3+, Ti.sup.4+, Zn.sup.2+, Sn.sup.4+, Ca.sup.2+, Mg.sup.2+, and Cr.sup.3+ and a retarding anion selected from the group consisting of acetate, nitrilotriacetate, tartrate, citrate and phosphate.
U.S. Pat. No. 3,833,061 discloses a method for selectively reducing the permeability of an oil-wet subterranean formation penetrated by at least one well bore by passing an oxidizing agent through and into contact with the formation for oxidizing and removing hydrocarbons from the surfaces of the formation and thereafter contacting the treated formation surfaces with a crosslinked polymer. The patent discloses the use of aluminum citrate as a crosslinking composition.